This invention relates to a process for making heat-curable prepregs from cellulosic fibers and to composites obtained by heat-curing such prepregs.
Laminated products prepared from heat-curable cellulosic "prepregs" are used industrially, for example in the production of baseboards for printed electrical circuits. These fibre-reinforced materials are commonly made by impregnating cellulosic fibres with an aqueous solution of a phenol-formaldehyde resin, heating the fibres to dry them, re-impregnating the fibres with a solution of an epoxide resin in an organic solvent, and then drying to form a heat-curable prepreg. Lastly, the prepreg is heated under pressure (usually in the form of stacked layers) to cure the epoxide resin and so form a composite.
The reason why it is necessary to employ two separate impregnation and drying cycles is that epoxide resins are, in general, too hydrophobic to impregnate cellulosic fibres directly; the fibres have first to be rendered less hydrophilic by treatment with an aqueous solution of a phenolic resin.
Because of the need for two impregnation stages and two drying stages the process is relatively costly, slow and complicated to carry out. It also has the disadvantage that an organic solvent is needed for the second impregnation, involving possible risks of toxicity and flammability and also costs of recovery of the organic solvent.
We have now found that these disadvantages can be substantially overcome by use of a process involving a single impregnation stage with a water-borne, heat-hardenable formulation containing an epoxide resin together with a member of a certain class of photopolymerisable resin.
GB No. 1 006 587 discloses adducts of diglycidyl ethers with (meth)acrylic acid. Amongst the adducts disclosed are those of formula EQU CH.sub.2 .dbd.CHCOOCH.sub.2 CH(OH)CH.sub.2 --O--ROCH.sub.2 CH(OH)CH.sub.2 O--.sub.n ROCH.sub.2 CH(OH)CH.sub.2 OCOCH.dbd.CH.sub.2 I
where R is a divalent aliphatic radical derived from an alkanediol or a polyalkylenediol and n is 0 to 20. The adducts are heat-polymerised and cured using peroxide initiators.
GB No. 1 139 100 discloses polymerisable products, having terminal unsaturation and being free of unreacted epoxy groups, prepared by reacting (a) 1 mol. of a diepoxide, (b) 0.5 to 0.9 mol. of a dicarboxylic acid, and (c) 1.0 to 0.2 mol. of acrylic, methacrylic or crotonic acid, or a half-ester of fumaric or maleic acid. The sequence of reaction preferably involves initial reaction between (a) and (b) to produce an advanced epoxide resin and then reaction of the latter with the unsaturated monocarboxylic acid. The epoxide resin (a) is a diglycidyl ether of a dihydric phenol or alcohol. The products are polymerised and cured using peroxide initiators.
GB No. 1 362 906 discloses adducts of (meth)acrylic acid with epoxide resins, obtained by advancing an epoxide resin with a carboxyl-terminated polyester. The starting epoxide resin may be a diglycidyl ether, a diglycidyl ester, or a N-heterocyclic polyglycidyl compound such as an N,N-diglycidylhydantoin. The (meth)acrylate-terminated adducts can be polymerised using free radical initiators and are useful in the production of moulded articles.
GB No. 1 367 207 discloses adducts of n moles of (meth)acrylic acid with 1 mole of a polyglycidyl compound of formula ##STR3## where A is a radical containing at least one group of formula ##STR4## wherein Z is a divalent radical required to complete a 5- or 6-membered heterocyclic ring, X is --H or --CH.sub.3, and n is 2 or 3. Preferred polyglycidyl compounds include diglycidyl derivatives of hydantoins. In Examples, diacrylates are prepared by advancing 1-glycidyl-3-(2'-glycidyloxy-n-propyl)-5,5-dimethylhydantoin with sebacic acid and reacting the advanced resin with acrylic acid. The adducts can be polymerised by means of free radical initiators.
GB No. 1 399 135 discloses the curing of these adducts with ionising radiation. This patent has a generic disclosure regarding the use of advanced epoxide resins, viz., that polyglycidyl compounds containing more than two of the heterocyclic residues can be prepared by reacting dicarboxylic acids with diglycidyl compounds in the appropriate molar ratio.
GB No. 1 400 286 discloses the curing of the (meth)acrylate adducts of GB No. 1 362 906 with ionising radiation.
GB No. 1 456 486 discloses air-drying, photocurable coating compositions based on a product obtained by reacting at least 60% of the epoxide groups of a polyepoxide with 0.01 to 0.5 NH-equivalent, per epoxide equivalent, of ammonia, an aliphatic or cycloaliphatic primary or secondary amine or a mixture of these compounds, and subsequently with 0.99 to 0.5 carboxyl equivalent of acrylic and/or methacrylic acid. Specified polyepoxides include glycidyl ethers of polyhydric alcohols (including 1,4-butanediol) and phenols, and polyglycidyl esters; aromatic polyepoxides are preferred. The epoxide-ammonia and epoxide-amine products are said to be 2-hydroxypropyl ethers containing free epoxide groups. The coating compositions are said to harden very rapidly, even in the presence of atmospheric oxygen, especially using .alpha.-substituted benzoins as photoinitiators, and to be particularly suitable for coating paper and cardboards.
GB No. 1 489 425 discloses photocurable resin compositions comprising a modified epoxide resin and a photosensitiser. The modified epoxide resin has a molecular weight of 700 to 5000, has polymerisable unsaturated residues linked to the main chain via ester bonds so as to provide the resin with an unsaturation equivalent of 200 to 3000, and also has --COOH groups linked to the main chain to provide the resin with an acid value in the range 30 to 150. One of the specified preparative routes involves reaction of an epoxide resin with a dicarboxylic acid to produce an advanced resin, which is then reacted with (meth)acrylic acid to give a (meth)acrylate-terminated resin, which is in turn reacted (via its --OH groups) with a dicarboxylic acid anhydride. Suitable advancing dicarboxylic acids mentioned include various saturated and unsaturated aliphatic acids. In an Example, a bisphenol A diglycidyl ether is advanced with adipic acid and the advanced resin is reacted with acrylic acid to give an intermediate product.
GB No. 1 521 933 discloses resins which are soluble in water before exposure to actinic radiation but which, on exposure to actinic radiation, polymerise and become insoluble in water, the said resins having the general formula ##STR5## where a is an integer of average value of at least 1, but preferably not more than 100,
each R and R.sup.1 represents a group of formula ##STR6## each R.sup.2 represents a hydrogen atom or a group of formula --(CH.sub.2 NH).sub.d COC(R.sup.6).dbd.CH.sub.2, with the proviso that at least 1, and preferably at least 25%, of the 2a groups R.sup.2 are other than a hydrogen atom, PA1 each R.sup.3 denotes an alkyl group of 1 to 4 carbon atoms, or conjointly each pair represents a group of formula --CH.sub.2 CH.sub.2 --, ##STR7## R.sup.4 represents a divalent aliphatic, cycloaliphatic, or araliphatic radical of 1 to 8 carbon atoms, especially an alkylene group of 1 to 6 carbon atoms, PA1 b, c, and d are each zero or 1, PA1 R.sup.5 represents a straight or branched chain aliphatic group of 2 to 20 atoms, such as one of 2 to 9 carbon atoms, especially an alkylene group which may be interrupted by one or more carbonyloxy groups or by one or more ether oxygen atoms, or, providing each c is 1, it may alternatively represent a group of formula ##STR8## R.sup.6 represents a hydrogen atom or an alkyl group of 1 to 4 carbon atoms, and PA1 R.sup.7 and R.sup.8 each represent a hydrogen atom or a methyl or ethyl group. PA1 (a) a photopolymerisable resin, and PA1 (b) an epoxide resin, PA1 e is an integer of at least 1, PA1 each R.sup.14 denotes a hydrogen atom or a methyl group, PA1 each R.sup.15 represents the divalent residue of a compound having two glycidyl groups directly attached to an atom or atoms of oxygen, nitrogen, or sulphur, after removal of the said two glycidyl groups, PA1 each R.sup.16 represents the divalent residue of a dihydric alcohol, a dihydric phenol, a dicarboxylic acid, or a compound containing two groups selected from amide groups and imide groups, after removal of two terminal hydrogen atoms attached to an atom or atoms of oxygen or nitrogen, PA1 each R.sup.17 denotes an alkylene group of 1 to 6 carbon atoms, an alkenylene group of 2 to 10 carbon atoms, an arylene group of 6 to 10 carbon atoms, or a cycloalkylene or cycloalkenylene group of 5 to 8 carbon atoms, PA1 each R.sup.18 denotes a divalent aliphatic, cycloaliphatic, or araliphatic group of 2 to 8 carbon atoms, PA1 with the proviso that at least 25% of the total of the (e+1) R.sup.15 and e R.sup.16 groups each represent a group of formula ##STR11## or a group of formula EQU --O(OC).sub.d --R.sup.20 --(CO).sub.d --O-- X PA1 wherein R.sup.4 has the meaning previously assigned, c and d are each zero or 1, PA1 each R.sup.19 denotes an alkyl group of 1 to 4 carbon atoms or each pair of R.sup.19 conjointly represents a group of formula ##STR12## R.sup.20 represents a straight chain or branched chain aliphatic group of 2 to 20 carbon atoms or, provided each d is 1, it may alternatively represent a group of formula ##STR13## and R.sup.21 and R.sup.22 either each denote a hydrogen atom or a straight or branched alkyl group of up to 9 carbon atoms or together they denote a tetramethylene, pentamethylene, methylpentamethylene, or hexamethylene group.
GB No. 1 537 909 discloses UV-crosslinkable urethane resins containing vinyl and carboxy groups which are prepared by reacting an epoxide resin with (meth)acrylic acid so that 60% to 100% of the epoxide groups are converted into .beta.-hydroxyester groups, converting some of these hydroxy groups into the corresponding urethanes by reaction with an isocyanate, and reacting the remaining hydroxy groups with a dicarboxylic acid anhydride. The epoxide resin may be pre-advanced by reaction with e.g., aliphatic or aromatic diacids. In an Example, a diglycidyl ether of bisphenol A is advanced with adipic acid and then reacted with acrylic acid to give an intermediate product.
EP No. 0 008 837 discloses a radiation-curable liquid coating composition based on (a) an epoxide resin containing at least one (meth)acryloyl group, (b) a photoinitiator, and (c) a crosslinking agent for the epoxide groups in (a). The resin (a) may be obtained by reacting an epoxide resin with a deficiency of (i) (meth)acrylic acid or (ii) a half ester of a hydroxysubstituted (meth)acrylate and a polycarboxylic acid or anhydride. The epoxide resin may be a hydantoin-based resin. Many types of crosslinking agent (c) are specified, those which are active at room temperature being preferred. In a special embodiment, an emulsifier is added to the resin (a) so that it can be diluted with water when mixed with a suitable crosslinking agent such as polyaminoamide. Curing of the composition is effected in two stages: in the first stage it is irradiated with UV light, and in the second, cure is completed by reaction of the epoxide groups with (c).
EP No. 0 030 213 discloses a photocrosslinkable layer which is water-developable after exposure, comprising a crosslinkable unsaturated monomer or oligomer, based on (meth)acrylic acid-modified epoxide resins with an acid value of less than 0.2, and a photoinitiator. There is used in an Example an adduct of acrylic acid with an epoxide resin obtained by advancement of neopentyl glycol diglycidyl ether with bisphenol A.
U.S. Pat. No. 4,309,529 discloses water-dispersible energy-curable polyesters having hydantoin groups and "backbone" alpha-methylene groups (.dbd.CH.sub.2). These polyesters have repeating units of formula: ##STR9## where R.sup.9 is a single bond or an alkylene group optionally having one catenary oxygen, R.sup.10 is --H, --COR.sup.12 or --CONHR.sup.13, R.sup.11 is --H or --CH.sub.3, wherein R.sup.12 is alkyl or alkenyl optionally substituted by phenyl or carboxyl and R.sup.13 is aliphatic or aromatic hydrocarbyl, W is a divalent group required to complete a 5- or 6-membered ring, Q is the divalent or trivalent residue of hydrocarbyl di- or tri-carboxylic acids formed by the removal of active hydrogens from the --COOH groups, containing up to 40 carbon atoms and optionally containing catenary oxygen atoms, Z is --COOH, and a is 0 or 1, but can be 1 for no more than 20 mole % of the acids.
In the principal statement of the invention, it was specified that at least 10 mole % of the acids from which Q is derived should have alpha-methylene groups. The polyesters are generally prepared by reaction of a heterocyclic diepoxide with a dicarboxylic acid; when an excess of diepoxide is used, polyesters having terminal epoxide groups are obtained, which groups can be reacted with (meth)acrylic acid. Many diglycidyl derivatives of hydantoins are suggested for use as the heterocylic diepoxide. Specified dicarboxylic acids include many saturated and unsaturated aliphatic and aromatic acids. For the polyesters to contain alpha-methylene groups, at least part of the dicarboxylic acid component must be an alpha-methylene acid. Some of the polyesters prepared in the Examples are acrylate-terminated.